1. Field of the Invention
This invention relates to a local oscillating circuit device for use in converters and measuring equipment for satellite broadcasting receivers.
2 . Related background Art
Accompanying the recent rapid development of information network systems, the demand for satellite communication and broadcasting systems has rapidly increased, and frequency bands are becoming higher. In order to satisfy these requirements, various circuit elements such as a field effect transistor, etc. which can be operated in a high frequency band have been studied and developed. A Schottky-barrier type electric field effect transistor (MESFET) formed of compound semiconductor has been used such as a field effect transistor for high frequency band. In addition, to reduce the size of the systems, reduce their prices and improve their performance, recently the integration (MMIC: Microwave Monolithic Integrated Circuit) of downconverter first-stage amplifiers for converting higher frequencies to lower frequencies is being advanced.
But in the local oscillators for use in converters and so on for satellite broadcasting receivers, the usual GaAs MESFETs having the active layer formed by ion implantation have been conventionally used, and GaAs MESFETs which are structurally arranged so that the bias voltage is optimized especially so as to make phase noises smaller have not been used.
Such conventional oscillators, using the above-described usual GaAs MESFET has too large phase noises as it is. This is due to the "low Q" which is intrinsic to the GaAs FET. The phase noise of the oscillator depends on various factors, and it is known that the phase noise much depends especially on low-frequency noises of the FET (which is one of the noises of 1/f because the spectrum is inversely proportional to a frequency), and the bias-dependency in further much also depends on the gate capacity Cgs. In view of this, to achieve low-phase noises, conventionally a dielectric resonator of a high dielectric constant has been externally added to obtain a required resonance spectrum. This is true with the MMIC resonator, and consequently the merits of the MMIC, such as miniaturization of the system, low costs thereof and so on, have not be able to be used.